The present invention relates to a device for depositing dead rock (overburden) of the type produced during surface or strip mining; the devices of the type to which the invention pertains, also being known under the term "cross-pit conveyor." Equipment of the type to which the invention pertains is to be used in installations, wherein in a two-stage or multi-step operation the dead rock or overburden, situated above useful ores and minerals, are removed and prebenched so that a drag line excavating the lowest stage can be restricted to that task, the drag line depositing overburden on the other side of the pit from which the useful minerals are extracted.
Strip mining of the type to which the invention pertains increases in usefulness particularly for mining anthracite and bituminous coal, soft coal, phosphate, oil sand, oil shale, and the like. Strip mining can be expected to increase in the future.
One has to distinguish between continuous and discontinuous strip mining operations. The continuous strip mining operation is characterized by the utilization of bucket wheel excavators, conveyor belts, and depositing equipment. The dead rock or overburden situated about the useful strata is taken up by the bucket wheel excavators which are movable through caterpillar type vehicles. Known equipment, moreover, transports the overburden through conveyors around the excavation pit, and depositing equipment, likewise movable on caterpillars, deposits the overburden into those areas adjacent to the excavation pit from which the useful minerals and ores or the like have already been removed. The conveyors used for that purpose are quite long, and all parts must be movable in order to follow the progress of the strip mining operation. Investment costs for the conveyors, and particularly their maintenance, are very high.
The so-called "drag lines" with a walking mechanism have been used in the past for directly depositing the overburden on the other side of a pit, which is sometimes several kilometers long. The boom of the drag line, suitably oriented, throws the overburden onto the other side of the pit. The drag line, moreover, exposes the useful strata in the bottom of the pit and the useful minerals are excavated by a second excavating system, consisting, for example, of rope-type stripping shovels and heavy trucks for removing the excavated and useful material. The boom length of the drag line has to be sufficiently large so that the overburden can be deposited in that portion of the mine from which the useful strata have already been removed. A conventional dimension for such drag lines, usually with a walking mechanism, is characterized by a boom length of from 80 to 100 meters, i.e., in excess of 300 feet, at a content of the excavating buckets from 40 to 60 cubic meters. The digging depth of a drag line determines also the length of a working cycle and should not exceed 30 to 40 meters, depending upon the boom length.
Since the cost of mining by means of a drag line and, here particularly, the cost for removing the overburden is below he cost of other systems, it is desirable to use drag lines even in those situations where the strata thickness of the overburden is larger than the depth that can be usually handled by drag lines. Conceivably one could remove overburden in the first step by means of bucket wheel excavators and remove the thus excavated overburden by means of conveyors, running around the pit and depositing the material on the other side of the pit. In particular, the overburden so removed could be deposited on top of overburden which a second stage drag line has already deposited. Aside from a rather high cost of this method, it offers the disadvantage that these two different types of strip mining systems or subsystems require a certain "tuning," and their cooperation is, in practice, really not very flexible; very accurate, extensive, and particularly consistently realizable planning is required.
Conveyor bridges are also known for the removal of overburden which are supported in a bridge-like fashion to both sides of the strip mining pit. These bridges are highly immobile. The are also very heavy and have to be adjusted and designed in order to accommodate the particular depth and width of the various pits and portions thereof; all of these requirements render their utilization rather unwielding.
Occasionally, one uses those types of depositing equipemnt as they are known from continuous strip-mining operations, and they are used particularly with extended booms for directly depositing the overburden without an extensive conveyor belt path in between. It has to be said, however, that the boom lengths available here are insufficient, and particularly one cannot use them together with existing drag line equipment. Generally, it should be said that boom lengths of up to 200 meters, or maybe even more, are required while normal strip mining machines operate with 60-to-100-meter boom lengths while cross-pit conveyors as they are used presently have a boom length of from 130 to 150 meters.
If one would extend the known cross-pit conveyors to accommodate a boom length of 200 meters or more under utilization of currently practiced construction techniques, one would obtain an extremely expensive piece of equipment which is difficult to operate and to install, and the operation would be rather time-consuming. They hardly could be dismantled for moving to another mining site for being reused which, of course, is a necessity for economic reasons. This deficiency is due to the fact that heretofore employed construction principles for cross-pit conveyors are characterized by an upper structure with booms and a rather long and elevated counterboom carrying a counterweight for compensating the long extension and the mass of the principal conveyor boom. Moreover, a turning connection is provided for pivoting the entire upper structure relative to a lower support structure. The lower structure is usually of a triangular configuration and the corner points are all affixed to a caterpillar vehicle. The overburden is transported through a particular bridge to the center of rotation of the device. The bridge is additionally supported at its other outer end by another caterpillar vehicle.
The rotating or turning connection as between the upper and lower construction portion of the cross-pit conveyor is by and in itself very expensive. Many construction parts are under a heavy bending load and a solid-wall-type carrier construction is needed, which by and in itself is very heavy. Moreover, the upper portion should be amenable to turning without or very little constraint as it is necessary for normal continuous strip mining. This, in turn, leads to a very complicated and expensive carrier sytem for the upper construction portion of the cross-pit conveyor. The center of gravity of such equipment is situated very high, which is not of advantage from a stability point of view.